Droplet spray generation device

ABSTRACT

An electronic drive system for a droplet spray generation device has a droplet generator with a perforate membrane driven by a piezoelectric transducer. An electronic circuit controls a power supply to control the charging of a capacitor to supply a drive signal to the piezoelectric transducer. The electronic circuit is arranged to control the operation of the power amplifier at substantially its resonant frequency.

BACKGROUND

The present invention relates to an electronic drive system for dropletspray generation and, more particularly to an electronic drive systemfor use in a droplet spay generator for an air-freshening device.

An aim of the present invention is to provide a low cost electronicdrive system for a droplet generator using a micro-controller toimplement signal generation and timing elements of a circuit to drivethe droplet generator.

Spray generators of the general type to which the present inventionrelates may, for example, be of the type described in EP-A-0615470.

WO-A-2005/097348 describes an electronic drive system for a dropletspray generation device of the type having a droplet generator includinga perforate membrane driven by a piezoelectric transducer, theelectronic drive system comprising a programmable micro-controller, apower supply for converting, in use, a battery supply voltage to powerthe device, a power amplifier connected to receive electric power fromthe power supply and supply a drive signal to the piezoelectricgenerator in use; and wherein the micro-controller is also arranged tocontrol the operation of the power amplifier, including the drive signaloperating frequency at substantially its resonant frequency, to measurethe current provided to the power amplifier by the power supply at aplurality of different frequencies, to determine, as the resonantfrequency of the droplet generator, the frequency at which the maximumpower is consumed by the amplifier, and to set the drive signaloperating frequency at the resonant frequency. Such a system effectivelytherefore determines the resonant frequency to use based on currentmeasurement.

SUMMARY OF THE INVENTION

However, if increased design flexibility (for instance to enable lowercost electronics) is required then, according to the present inventionthere is provided an electronic drive system for a droplet spraygeneration device, the drive system comprising a droplet generatorhaving a perforate membrane driven by a piezoelectric transducer; anelectronic circuit controlling a power supply to control the charging ofa capacitor to a supply voltage providing power to a power amplifierconnected to receive electric power from the capacitor and supply adrive signal to the piezoelectric transducer in use; and wherein theelectronic circuit is also arranged to control the operation of thepower amplifier including the drive signal operating frequency atsubstantially its resonant frequency, where the resonant frequency iscalculated by measuring and/or timing changes to the voltage across thecapacitor when the droplet generator is operated in a plurality ofbursts at different frequencies, by charging the capacitor to a givenvoltage, and either actuating the droplet generator for a given time andeither measuring the voltage drop across the capacitor at the end of thegiven time or the time taken to recharge the capacitor to the set valuethereafter, or actuating the droplet generator and measuring the timetaken for the voltage to fall to a known value, and determining as theresonant frequency of the droplet generator, the frequency at whichrespectively the time to recharge is the greatest, or the voltage dropis the greatest, or the time taken for the voltage to fall to the knownvalue is shortest, and to set the drive signal operating frequency atthe resonant frequency thus determined.

The frequency search algorithm may determine the resonant frequency ofthe spray head at which maximum power is consumed by estimating thepower consumed at each test frequency by charging the power supplycapacitor to a set voltage, switching the output on for a set time, andthen measuring the time taken for the capacitor to be charged up toanother set voltage. The test frequency at which the longest rechargetime is measured is then set as the resonant frequency.

A second method to estimate the power consumed at each test frequency isto charge the power supply capacitor to a set voltage, to switch theoutput on for a set time, and then to measure the capacitor voltage. Thetest frequency at which the lowest voltage value is measured is then setas the resonant frequency.

A third method to estimate the power consumed at each test frequency isto charge the power supply capacitor to a set voltage, to switch theoutput on until the capacitor reaches a second set voltage, and torecord the time taken for the capacitor to reach this second voltage.The test frequency at which the shortest time is then set as theresonant frequency.

Such a system also enables a wider choice of DC/DC converter design tobe employed within the power supply. For example designs using pulsewidth modulation, pulse frequency modulation and trickle chargers may beused.

By using a micro-controller the components of the present invention costless than the expected cost of a device using a custom applicationspecific integrated circuit (ASIC) providing the same functionality inall but very high production volumes.

The power amplifier preferably comprises a resonant bridge circuit andthe micro-controller outputs in use a pair of signals in anti-phase tothe resonant bridge circuit to control the amplifier.

The drive system may be used to power and control a droplet generatorfor an air freshening device, in which case, preferably, the electronicdrive system includes a multi-position switch connected to themicro-controller to control a timer provided by the micro-controller toset the interval at which the drive signal is provided to the poweramplifier to control the interval at which the droplet generator isautomatically operated.

The micro-controller may also provide a second timer to generate thedrive signal, on demand, as a plurality of pulses of fixed interval overa predetermined period shorter than the interval at which the dropletgenerator is automatically operated, the second timer being operated bya mono-stable switch connected to the micro-controller.

The power supply is preferably controlled by the micro-controller anduses an integrated pulse frequency mode.

By using a micro-controller device and using just a single pin tointerrogate the multi position and mono-stable switches amicro-controller with only six I/O pins may be used.

The power available from batteries is maximised by using themicro-controller's “sleep” mode, but waking it up under the control of atimed “watchdog” interrupt and whenever the mono-stable switch ispressed.

BRIEF DESCRIPTION OF THE DRAWINGS

An example of the droplet spray generation device incorporating a drivesystem according to the invention will now be described with referenceto the accompanying drawings in which:

FIG. 1 illustrates a cross-sectional view of the droplet generator;

FIG. 2 is a block diagram of the device;

FIG. 3 is a signal diagram illustrating various voltages during theoperation to establish the resonant frequency; and,

FIG. 4 is a UML “activity” diagram illustrating the process followed bythe micro-controller to determine the resonant frequency.

DETAILED DESCRIPTION

As seen in FIG. 1 and FIG. 2, an air-freshening device 10 is providedfor spraying air-freshening fluid through a spray head 11. The sprayhead 11 comprises a perforate membrane 32 coupled to an annularpiezoelectric transducer 31 mounted on a substrate 33. When thetransducer is activated, air-freshening fluid is sprayed from areservoir (not shown) through the perforate membrane 32 to the outsideenvironment.

The block diagram of FIG. 2 shows the spray head 11 controlled by amicro-controller 20 (explained in more detail below) and is powered bytwo “AA” alkaline batteries 12 providing in the range of 2-3.2 volts.Micro-controller 20 is, for example, an Atmel “ATmega168” with 32 I/Opins and operating at 10 MHz. This contains all of the functional blocksrequired to implement the features described here.

In an air-freshener of this type, there may be a requirement that thedevice 10 sprays fluid through the spray head 11 in two different modes.A background mode is therefore provided wherein a preset quantity offluid is emitted by the spray head 11 at intervals determined and set bya user. This is achieved by actuating the piezoelectric transducer for apredetermined length of time. The piezoelectric transducer is operatedat a predetermined AC voltage at a suitable frequency within apredetermined range, in the present example 75-90 kHz. A slide switch 13allows the user to control how often the spray head 11 emits fluid. Theslide switch 13 is a five-position slide switch that the user adjusts toset the time interval between background sprays.

A boost mode is also provided; in this mode, when the user operates pushswitch 14 the spray head sprays a preset quantity of fluid. Typically,the background spray emits a maximum of 400 mg of fluid per day and theboost mode emits 10 mg of fluid in 10 seconds on activation of the pushswitch 14.

The air-freshening device 10 is constructed to be power efficient andthe device contains a number of features to achieve this. A power supplyunit (PSU) 21 provides power for the components on the micro-controller20, selectively as determined by the micro-controller and indicated byswitching the output 211 of the micro-controller. The PSU 21 includes apair of DC-to-DC converter integrated circuits together with theinductor, diode and capacitors they requires to operate (not shown). Oneof the DC-to-DC converter integrated circuits is arranged to supply 5volts from the two “AA” cells 12 to the micro-controller 20. It has verylow quiescent power consumption when not under load. To minimise powerconsumption, the microprocessor is kept in “sleep” mode when notspraying. The PSU 21 also includes a second DC/DC converter whichprovides a 6 volt signal to a capacitor 22 which is charged to 6 voltsand which supplies energy at a voltage (V_(cap)) to the power amplifiercircuit 23 to drive the spray head 11.

The power amplifier circuit 23 which provides the drive signal to thespray head 11 is configured as a resonant bridge circuit. It requirestwo digital drive signals in anti-phase at a desired frequency.Operating from the 6-volt power supply, it can generate a drive signalof the order of 40 volts peak-to-peak. A series capacitance of the sprayhead 11 forms part of the resonant bridge power amplifier circuit.

As an alternative, a resonant power amplifier circuit incorporating atransformer and transistor switching device can be used. This amplifierconfiguration would require only a single digital drive signal from themicroprocessor. This amplifier may include an inductor to match thetransformer output to the spray head load.

The background timer 25 is used to set the time interval between the“background” sprays. It also sets the time duration of each spray. Thetimer 25 is provided by software on the micro-controller 20. The slideswitch 13 is connected to the background timer 25 to allow the user toset the interval between each background spray.

Boost timer 24 is also provided through software on themicro-controller. When push switch 14 is pressed, the software executesthe boost spray. It splits the fluid to be sprayed into a number of“quanta” of fixed length, with a fixed time interval between each. Inthis example, each quanta is 1 second, which is repeated 10 times with a200 ms time gap between each pulse. The 200 ms gap lowers the averageflow rate and gives the spray head 11 time to recover between eachspray.

The micro-controller 20 has a stable 10 MHz internal frequency generator26 that is used to generate square wave drive signals at the frequenciesrequired, and with the frequency resolution required to drive the sprayhead 11. By adjusting the number of CPU clock cycles between outputtransitions, frequency generator software can generate square waves inthe range of 75-90 KHz (which is required for the spray head 11), with aresolution of the order of 1 KHz. Since every other frequency step is anodd number of clock cycles, the duty cycle of the square wave generatedis only approximately 50% in these cases. In addition, themicro-controller 20 is programmed to generate dual signals inanti-phase, overlapping by one clock cycle. These overlapping drivesignals are required for the power amplifier 23 that drives the sprayhead 11.

The spray head 11 operates best at its resonant frequency which is inthe range of 75-90 KHz, and requires a drive voltage in the order of 40volts peak-to peak at this frequency. When driven at its resonantfrequency, the power consumption of the transducer is at a maximum. Todetermine this resonant frequency, the time taken for the capacitor 22to recharge to the supply voltage is measured as follows below.

A high impedance voltage divider is disposed across the capacitor 22 andis designed so that its output voltage is 1.22 volts when the capacitor22 is fully charged. The voltage divider output (which may be bufferedby a suitable low power op-amp, not shown,) is connected to one input ofthe micro-controller's on-chip comparator and the other comparator inputis connected to the micro-controller's on-chip 1.1 volt reference.Software running on the micro-controller is thus able to detect when theoutput of the voltage divider exceeds 1.1 volts and this indicates thatthe capacitor 22 is close to being fully charged.

In order to set the operating frequency, the software on themicro-controller measures the time to re-charge of the capacitor andoperates the circuitry as follows and as shown in FIG. 3 and FIG. 4. Foreach of a number of test frequencies f of the voltage (V_(load)) appliedto the spray head 11, the micro-controller executes a routine asfollows:

-   -   At time t=0, it switches on the PSU (V_(psu)).    -   It waits for 1 millisecond, known to be greater than the maximum        time that the capacitor takes to re-charge.    -   At time t=0.8 ms, it switches off the PSU (V_(psu)).    -   At time t=1.0 ms, it switches on the amplifier (V_(load)) to        cause the spray head 11 to spray at the particular test        frequency f for 1 millisecond.    -   At time t=2.5 ms, it switches on the PSU (V_(psu)).    -   Using the on-chip comparator, it looks for six consecutive        voltage divider readings greater than 1.1 volts. This helps to        reject noise spikes on the voltage divider output and avoid        measurement errors.    -   The micro-controller then stores the value of time to reach this        point and the related test frequency f.    -   If the system has just been switched on, then the routine        immediately repeats for each of the test frequencies. Once the        system has started operating normally, at appropriate intervals        until all the test frequencies have been used, a test spray is        carried out at one of the frequencies in the range.    -   In both cases, once all the frequencies of f have been tested,        the micro-controller then sets the operating frequency to be the        test frequency f corresponding to the longest capacitor recharge        time.

FIG. 3 illustrates the timing of the PSU voltage V_(psu), the loadvoltage V_(load) applied to the power amplifier 23, and the voltageV_(cap) output by the capacitor (to which the voltage divider outputvoltage is directly related.) Three graphs of the capacitor outputvoltage V_(cap) are shown, to illustrate variation of the voltageV_(cap) with changing frequency and hence changing capacitor rechargingtime, t_(f1)-t₀, t_(fn)-t₀, t_(fres)-t₀, for three different testfrequencies, f1 (a first test frequency), fn (an nth test frequency),and fmax (the test frequency established as the resonant frequency).

It has been found that test sprays with short (e.g. 6 millisecond) gapsbetween them are sufficient at start-up to enable an accuratemeasurement of the resonant frequency.

As mentioned above, the system has two calibration modes: When thesystem is switched on, the resonant frequency is unknown. So, beforespraying, all frequencies in the range are tested as described above tofind the desired operating frequency of the spray head 11. A fullcalibration measurement such as this makes an audible sound which mayprovide indicator of proper functioning to the user. When operating, theresonant frequency may to shift, due to variation of circuit parameterswith temperature, etc. Therefore, at regular intervals, a differentsingle frequency is selected and a test spray carried out. When allfrequencies in the range have been measured, the operating frequencyvalue is updated, and the process restarts. The sound made by such asingle point measurement is virtually inaudible due the calibrationupdate being over many background sprays.

A voltage-measuring solution would be similar to the system describedabove, with the central timing loop replaced by a voltage measurementvia the micro-controller's built-in analogue to digital converter. Thealgorithm would search for the lowest voltage, measured just after the 1millisecond test spray pulse.

1. An electronic drive system for a droplet spray generation device, thedrive system comprising a droplet generator having a perforate membranedriven by a piezoelectric transducer; an electronic circuit controllinga power supply to control the charging of a capacitor to a supplyvoltage providing power to a power amplifier connected to receiveelectric power from the capacitor and supply a drive signal to thepiezoelectric transducer in use; and wherein the electronic circuit isalso arranged to control the operation of the power amplifier includingthe drive signal operating frequency at substantially its resonantfrequency, where the resonant frequency is calculated by measuringand/or timing changes to the voltage across the capacitor when thedroplet generator is operated in a plurality of bursts at differentfrequencies, by charging the capacitor to a given voltage, and eitheractuating the droplet generator for a given time and either measuringthe voltage drop across the capacitor at the end of the given time orthe time taken to recharge the capacitor to the set value thereafter, oractuating the droplet generator and measuring the time taken for thevoltage to fall to a known value, and determining as the resonantfrequency of the droplet generator, the frequency at which respectivelythe time to recharge is the greatest, or the voltage drop is thegreatest, or the time taken for the voltage to fall to the known valueis shortest, and to set the drive signal operating frequency at theresonant frequency thus determined.
 2. An electronic drive systemaccording to claim 1, wherein the power amplifier comprises a resonantbridge circuit and the electronic circuit outputs in use a pair ofsignals in anti-phase to the resonant bridge circuit to control theamplifier.
 3. An electronic drive system according to claim 1 whereinthe power amplifier comprises a resonant circuit using a transformer,transistor and impedance matching inductor.
 4. An electronic drivesystem according to claim 1, wherein the electronic drive systemincludes a multi-position switch connected to the electronic circuit tocontrol a timer provided by the electronic circuit to set the intervalat which the drive signal is provided to the power amplifier to controlthe interval at which the droplet generator is automatically operated.5. An electronic drive system according to claim 4, wherein theelectronic circuit provides a second timer to generate the drive signal,on demand, as a plurality of pulses of fixed interval over apredetermined period shorter than the interval at which the dropletgenerator is automatically operated, the second timer being operated bya mono-stable switch connected to the electronic circuit.
 6. A spraygeneration device including an electronic drive system according toclaim
 1. 7. An air freshener including an electronic drive systemaccording to any of claim 1.